A proton exchange membrane fuel cell (PEMFC) is a fuel cell that incorporates a polymer membrane, having a proton exchange function, as an electrolyte. The PEMFC may be operated at low temperatures, has high efficiency and high current and power density, and has a fast reaction time to a change in load while starting up in a short time, compared to other fuel cells. Accordingly, PEMFCs have been recently used as fuel cells for hydrogen-powered vehicles.
A PEMFC stack typically consists of hundreds of unit cells. Each of the unit cells is an electricity generating element for the fuel cell, and includes a membrane electrode assembly (MEA) formed by bonding anode and cathode electrodes to a polymer electrolyte membrane, a gas diffusion layer (GDL), and a separator. An electrochemical reaction takes place in the electricity generating element in order to produce electric power. The electrochemical oxidation of hydrogen as a fuel occurs in the anode of the MEA, and the electrochemical reduction of oxygen as an oxidant occurs in the cathode thereof. In this case, electrical energy is generated due to the movement of electrons generated by the reaction, the protons generated in the anode migrate to the cathode through the polymer electrolyte, and oxygen combines with the protons in the cathode to produce water.
Although a variety of research on polymer electrolyte membranes has been performed to date, NAFION® developed in the early 1960s is still widely used as a proton exchange membrane for a fuel cell. NAFION® is a branched polymer made by covalently bonding a sulfonic acid group to the side chain end of fluorine-substituted alkyl ether in the polymer main chain of fluorinated hydrocarbon similar to TEFLON®. Here, the sulfonic acid group is rehydrated by water molecules so that ionic conductivity is activated. That is, protons are able to freely move in the electrolyte due to water molecules present in electrolyte membranes, and high ionic conductivity is thus exhibited.
A hydrogen-powered vehicle may include a fuel cell stack, balance of plant components (an air compressor, a heat exchanger, etc.), a fuel supply device, an auxiliary power source, a motor, a motor controller, etc.
An MEA affecting the performance of a fuel cell stack must have a certain level of relative humidity over a wide range of operating temperatures. To this end, a humidifier may be provided in an air supply system outside the stack.
A gas-to-gas membrane humidification method is an external humidification method used to humidify a fuel cell stack for a vehicle. Since this method recovers and reuses moisture and heat discharged from the stack, it has an advantage in that separate energy sources or mechanisms are not required.
However, techniques for preventing permeation of gases other than moisture present in wet air are required in order for a gas-to-gas membrane humidifier to have moisture selectivity as its basic characteristic. Therefore, the above method is disadvantageous in a layout of an engine room due to an increase in cost necessary for an external humidifier and an increase in volume. In addition, since the membrane humidifier uses a polymer membrane, it is difficult to control a supply amount of water vapors at a given temperature. Furthermore, since a hydrogen supply system does not have a separate humidifier, it should rely on relative humidity produced in a cathode being inversely diffused to an anode.
Accordingly, there is a problem in that the lack of proper humidity at an operating temperature having low relative humidity may cause the dry-out of an MEA and the performance deterioration of a fuel cell. In particular, severe and long-term drying in the stack may cause irreversible damage to the MEA.
In addition, since a conventional MEA has a structure in which the side end of a polymer electrolyte membrane disposed between anode and cathode electrodes is exposed to the outside, water molecules are diffused to the outside from the side of the polymer electrolyte membrane, thereby corroding the exterior of the stack and deteriorating electrical insulation safety.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.